Production of primary ultramarine



May25, 194. w. J. KRUPPA ETAL.

PRODUCTION OF' PRIMARY ULTRAMARINE Filed July 24, 1945 Patented May 25,1948

PATENT (TFFICE PRODUG'IION OE 'PRIMABYeULT-RAMARIN E William J 'Kruppa, -Middlesex,- Ailing PE Beardsprimary ultramarine" and these reactions require different times "and different temperatures. In the process-.asordinarilyused' there. is little con- 'trol possible. Thecruciblesare stacked in" the furnace and thefurnace is then fired. The cruci- 'bles;nearest the'firetheat up most rapidly arid "reach top temperature'first. It. is therefore necessary toover-heat' the crucibles nearest thefire inorder. to bring'the more distant ones to a stifliciently high temperature, or else. a compromise "must be struck in vvhi'ch some. ofthe crucibles Company, New :York, N. :-.Y., a

.-Ma-ine Applicationfiuly 24,

"5 Claims.

This invention relates to an improvement in the 'pro'du'ction ofi ultramarine.

Up to now the standard commercial procedure forultramarine has involved the charging'of the raw mix. containing .china clay, e silica, sodaash, sulfur, and a carbonaceous reducing agent such as pitch or rosin, into ceramic pots *or crucibles of fairly small size; .which are then stacked into a single furnace which normally contains-more than 1,000 crucibles. 'Ihe turnace isthen "fired= :=up" to --a-certain temperature; held there for a suificient time to produce primaryultramarine 5 from the raw mix, w-hich is-a product that 'is'weak and dull inshade, followed by a-very slow cooling time-Whichtakesa couple=of-Weeks-, during which I oxygen' passes through the walls of the crucibles, which are of carefully controlled porosity, and i oxidizes the primary'ultramarine to the stronger and'brighter secondary ultramarine which is the pigment of commerce. *aboutone-seventh to one-eighth of the combined sodium in the primary ultramarine.

This operation removes -'In the*copending applications of Beardsley and whiting, Serial N0."606,887, filed July-24, 1945,

and Serial No, 606,888, filed July 24,"1'945,'.there;

- is described and -claimed an improved two=step process in which the production-of-primary ultramarine is efiected in substantiallyimpervious ceramic containers; such-ascrucibles," and then *the oxidation step is effected-"in'the same or"a' 'd-ifierent furnace un'der conditions 'whichpermit regulated control of the'contact of the" oxidizing gases with the-primary'ultramarine. 'In'Jone'Of said applications there is also "described and "claimed the-use of sulfur dioxide as" anoxidizing agent in the second step" oifthe process.

"The two processes have" the 1 first or. primary ultramarine firing in common. Various reactions appear" to take "place in the production 1 of -the near the fireare somewhat over-heated; the crucibles in the middle of -the'furnace areheate'dito the right temperature, .and the :more distant crucibles .are somewhat .uriderheated.

It..-has been, proposed, and r in some cases-pub corporation of 2 intovpracti'cei to use multiple hearths so thatthe distances that the" heating gases have to: pass :is less. I'Thisintroduces a-serious complicationsas it is: extremely difficult; in fact almost impossible, #5 to'control the individual fires; perfectly *even-ly. "The'procedure used in the past not only results in uneven temperaturesin the different-crucibles but the rate at""which: the temperature is raised cannot be accurately controlled. The whole 10' operation is a? necessary; although undesirable, compromisewith-preferred operating conditions.

According to the present invention the difficu-lties controlling temperature and time of -heating "are avoidedrby "using a continuous process-instead of a batch-process. "I-lhe crucibles -or=-other containers are charged onto a moving vehicle or conveyor and are moved continuously-- throughm furnace, which may advantageously be an adaption-of the tunnel kiln-used in the ceramicindustry. Gasesflowthrough .the'qki-lmincountiercurrent, which permits" very' emcientvheating, and -an--exa'ct control of rate 'of' heating and timeof d'etention in? zones of particular temperature is ob-tainedby regulating the len'gths'of the zones. The crucibles are preferable-in fairly-short rows across the car or conveyor so that very uniform heating-for each crucible is possible. -Becausenof the accurate i temperature control and possibility of rapid heatingz without =local over heating'z the time required for each" step in'the process--of forming" primaiy ultra-marine may be verydrastically reduced, resu ltingin animportant increase in output.

:After the= crucibles :have reached the-2 desired .35 top temperature they pass into a zone wherethe top' temperature is-maintained Without increase, T- and remain ?-in thissoa'king "or detention zone'the desired time. Fr-om-rthis section-they pass into the cooling zone. in this zone-air -is' circulated Ao around thewcruciblesin -adirection counter-to their movement, "absorbing heat which may be used attl'le other" end of the tunnelwto preheat theincomi-ngprucibles. .--Thisresults-in -a-*very material saving in fuel.

llunnel kil-ns havebeenlcnown foreyearsdn the ceramic industry. Their merits arewellestabdished but .they .are not suitable for. processes which. evolve corrosive-ornoxious gases-as is the case in the production of ultramarine Where volatile sulfur compoundsburn to sulfur-dioxideand sulfur. trioxide. This is one of thereasons-Why .ultramarine furnace practice has not changed -materiallye-in thevpast eighty; years. Batch-=iuranaces built-othrick or other refractory material, and -prov-ided w-ith powerful-{venting throuahthe stack which carries off combustion gases, have been considered necessary for ultramarine production. Tunnel kilns such as those used in the ceramic industry werenot considered suitable, and in fact they cannot be practically used in the formin which they are normally built and operated.

We have found that by two comparatively simple modifications it is possible to use tunnel kilns 1 with complete safety and high efliciency. It is practically impossible to design a tunnel kiln which is gas tight. We have found'that it is not at all necessary to do so. A standard construction of tunnel kiln may be employed in the firing of ultramarine provided that-'aslight negative pressure-is maintained in the kiln so that any small leaks, which are unavoidable in a practical kiln, will merely cause'a slight leakage of air into the kiln. Alternatively, if in certain zones of the kiln it appears desirable to carry a pressure greater than atmospherimthen those zones are surrounded with a housing in which an air pres sure slightly reater than that of the hot gases.

is maintained, as by a fan, for example. In one .or the other of these two Ways it is assured'that there will be a slight leakage of air into the kiln rather than of acid gases outward.

,The second modification results from the fact 'that the sulfur compounds in the ultrarnarine process burn to sulfur'dioxide, and in'the' high temperature zones the refractory acts as a low grade catalyst so that some sulfuric acid is formed. with the water vapor in the heating gases.

the corrosion problems are insuperable.

condensation of sulfuric acid Within the kiln can be avoided and no corrosion problems result. The low temperature portion of the kiln inwhich the crucibles are first heated up is, of course,

supplied with hot air from the cooling zone with only such boosting of temperatures by burners as may be necessary. 7 i V In the inlet'zone 'of the kiln, in which the crucibles experience their first heating by means of .hot air comingfromthe cooling zones, the

courseof travel of the heating gases is co-current with the crucible travel. Although the temperature. in this zone would below enough to permit the condensation of sulfuric acid from gas containing that substance; the co-current flow of hot air along this zone prevents the ene trance of sulfuric-bearing gases from the hotter part of thekiln into this zone. The kiln is designed so that the hot air coming in co-current direction from'the inlet zone meets the sulfuricbearing gases coming countercurrent'from the 'hot zones at the point where both of them have attained a temperature of about 300 C. At this point both types of gas are drawn off to the stack.

The temperature in the first heating zone is not high enough to cause the catalytic oxidation "of any sulfur dioxide produced there to sulfur trioxide. This zone is, therefore, free from any sulfuric acid produced there, and, as explained,

' 'gas frompthehot end which contains sulfuric acid cannotenter.

7 The present process permits a very material saving'in fuel cost over the furnaces used in ultra-marine plants at the present time. reason for this is that excellent recoveryof heat :by heat exchange between the crucibles which 1 are cooling and those'which are heating is pos- One 2,442,173 7 I r I 1' sible. For another thing, the usual type'of ultramarine furnace stores as much or more heat, in the brickwork of the furnace as in the crucibles and their contents. work is lost during cooling and must be replaced during the next firing. By. contrast, a continuous All this heat in the brick- T kiln, once heated, retains its heat except for the loss by radiation, etc.

Another important advantage of the present Fig. 1 is a diagrammatic plan View of a tunnel j kiln according to the present inventionLand Fig. 2 isa'vertical' section along lines 2-2'of If this acid condenses in any part of the kiln" How ever, we have found that by drawing off'the heating gases before they reach a portion of l the kiln which is at a temperature below 300 C.

Fig. 1.

The drawings show, in diagrammatic form, a

tunnel kiln, which,'in its general designyfollows accepted tunnel kiln practice as used in the ceramic arts. r

The kiln is shown at 1| provided with an-entrance door 2 and exit door 22. A'track 3 passes through'the kiln on which trackcars 23 travel.

The cars are of conventional design provided with a solid firebrick'insulating floor 2 1 anda superimposed hollow false floor 29 onwhich impervious ceramic crucibles 25 are carried introws .of four.

The crucibles containthe ultramarine' raw" mix.

temperature kilns,.the hearth wall 20 extends ,In'accordance'with common practice for hi h ,nearly to the firebrick floor 24 r the cariland the wheels of the car and other metal parts of the car are protected from hot gases escaping between the car floor'and the kiln walls by sand seals. 'These consist of sand beds 21 introughs, attached to the kiln walls ,into which beds metal angles 28' extend downwardly from thametal v frameworkof the cars. 1 r

The mechanism for moving the-cars slowly through the kiln is'conventional and is therefore not shown. I H 1' .The kilnniay, for example, be'about.2 l5 -to;300

feet long and the speed of travel, of the :car's therethroughis such that it takes approximately 24 hours for a car to pass through. The kiln is shown as divided into four zones, 'A, B, C and; D. The first, which is a preheating zone, carries-the temperature of the crucibles fromi15 to 300? C.

The heating zoneB carries the temperature from 300 to 750 0., which temperature islmaintained in the firing section C and then the crucibles are cooled down to about :100 C. in section D. In the cooling zone D cooling is eifecte'dl by a circulation of cold air'from the blower tc'ountern current to the cars andcrucibles moving through the ,zone.

' ample,'to about 700 C. at the hot end of zone D.

through the ducts 6 and I6 by. the blowersl and I1, which driverthe heatedkair through ftheair This airbecomes heated up, for ex-.

.At this 'point 5 the majorportionis remoyed 1 headers 8 and 18; The' blowers l and H are.

about evenly throughthe two ductsv 6' and lfiand their capacity is so chosehin.compariso rwith the capacity of the blower 4 thatthererexists in" zone D a very slight positive pressure;

The hot air in the headers B and lt'isjintr o' duced into zones A and B through the jpipes'M provided with valves '20. A portion oftheflair adjusted so that the stream of hot air divides V from zone D flows directly on into zone C where it is admixed with combustion gases from oil burners l3, fed from fuel headers l2 and controlled by valves 2|. The hot air from the headers 8 and I8 is so adjusted that the flow of air and combustion gases through zones C and B is countercurrent to the travel of the cars carrying the crucibles but is cocurrent in zone A.

Gases and air from zones A, B and C are sucked out through the duct 9 by the exhaust fan It! and blown into a stack II. The exhaust is so designed that there is a slight negative pressure in zones A and B and a neutral or slightly positive pressure in zone C. This positive pressure is only suflicient to permit satisfactory operation of the burners.

At the duct 9 the temperature of the cocurrent heating air in zone A and the countercurrent flow of gases in zones B andC is above 300 C. Therefore, at this point the sulfur dioxide which is evolved in zones B, C, a part of which is transformed into sulfur trioxide by the catalytic action of the refractory lining of the kiln and united with water to form sulfuric acid, is at a temperature sufficiently high so that liquid sulfuric acid does not precipitate. As a result there is no corrosive effect on the kiln itself, the stack ll being of suitable refractory material which is not adversely affected by sulfuric acid.

In zones A and B the pressure is negative. In zone C, however, the pressure is preferably slightly positive, and therefore this zone is surrounded with a casing [5 in which air is maintained by the blower 19 at a pressure higher than that inside the kiln. As a result any leakage will be from the atmosphere into the kiln and not the reverse, so that no hazard is encountered due to outward leakage of noxious gases.

While the rapid heating advantages of the kiln in producing primary ultramarine are enjoyed regardless of the nature of the crucibles in which the ultramarine raw mix is contained, it is preferable to use impervious ceramic crucibles so that no reactive gases encounter the raw mix during firing to primary ultramarine. The manufacture of ultramarine under conditions which prevent reactive gases from contacting the ultramarine mix is claimed in our copending application, Serial No. 606,888, filed July 24, 1945.

In the claims the term primary ultramarine" is used in the ordinary sense, namely, to cover the product obtained in the first firing stage of the reaction before oxidation. The term green is no longer descriptive of the primary ultramarine, as the crude fired ultramarine of presentday production is rarely green and that of our process, never so.

We claim:

1. A continuous process for the production of primary ultramarine from ultramarine raw mix, which comprises: continuously moving said mix through a preheating zone and preheating said mix to about 300 C. while said mix is moving therethrough; continuously moving the so preheated mix through a heating zone and gradually increasing the temperature of said mix to primary ultramarine firing temperature while said mix is moving therethrough; continuously moving the so heated mix through a firing zone and maintaining said mix therein at said firing temperature until the formation of primary ultramarine is complete; continuously moving said primary ultramarine through a cooling zone and cooling said primary ultramarine gradually therein; maintaining said ultramarine mix and said primary ultramarine substantially out of contact with the atmospheres of all said zones but in heat-exchange relationship therewith; continuously withdrawing the atmosphere from said preheating and heating zones at a point between said zones; and causing by means of this withdrawal, the atmosphere in said preheating zone to flow co-currently and the atmospheres in said heating and firing zones to flow counter-currently with respect to the motion of said ultramarine mix.

2. The process according to claim 1, in which the atmospheres withdrawn from the preheating and heating zones at a point between said zones are withdrawn at a temperature of about 300 C.

3. The process according to claim 1, in which the atmospheres of at least a portion of said heating and firing zones are maintained at a slightly negative pressure with respect to the external pressures of the atmospheres surrounding said zone.

4. The process according to claim 1, in which at least a major portion of the heat for said preheating zone is supplied by a co-current heating gas substantially free from sulfuric acid.

5. The process according to claim 1, in which the 'primary ultramarine firing temperature is about 750 C.

WILLIAM J. KRUPPA. ALLING P. BEARDSLEY. STANLEY H. WHITING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS FOREIGN PATENTS Country Date Great Britain 1890 Germany Feb. 18, 1902 OTHER REFERENCES Manufacture of Colors for Painting, by Rifiault et al., 1874, page 305.

Number Number 

